1. Field of the Invention
This invention relates to radiodiagnostic reagents and peptides, and methods for producing labeled radiodiagnostic agents. Specifically, the invention relates to specific-binding peptides, methods and kits for making such peptides, and methods for using such peptides to image sites in a mammalian body labeled with technetium-99m (Tc-99m) via a radiolabel-binding moiety which forms a complex with Tc-99m. In particular, the peptide reagents of the invention are covalently linked to a polyvalent linker moiety, so that the polyvalent linker moiety is covalently linked to a multiplicity of the specific-binding peptides, and the Tc-99m binding moieties are covalently linked to a plurality of the specific-binding peptides, the polyvalent linker moiety, or to both the specific-binding peptides and the polyvalent linker moiety.
2. Description of the Prior Art
In the field of nuclear medicine, certain pathological conditions are localized, or their extent is assessed, by detecting the distribution of small quantities of internally-administered radioactively labeled tracer compounds (called radiotracers or radiopharmaceuticals). Methods for detecting these radiopharmaceuticals are known generally as imaging or radioimaging methods.
In radioimaging, the radiolabel is a gamma-radiation emitting radionuclide and the radiotracer is located using a gamma-radiation detecting camera (this process is often referred to as gamma scintigraphy). The imaged site is detectable because the radiotracer is chosen either to localize at a pathological site (termed positive contrast) or, alternatively, the radiotracer is chosen specifically not to localize at such pathological sites (termed negative contrast).
A number of factors must be considered for optimal radioimaging in humans. To maximize the efficiency of detection, a radionuclide that emits gamma energy in the 100 to 200 keV range is preferred. To minimize the absorbed radiation dose to the patient, the physical half-life of the radionuclide should be as short as the imaging procedure will allow. To allow for examinations to be performed on any day and at any time of the day, it is advantageous to have a source of the radionuclide always available at the clinical site.
A variety of radionuclides are known to be useful for radioimaging, including .sup.67 Ga, .sup.99m Tc (Tc-99m), .sup.111 In, .sup.123 I, .sup.125 I, .sup.169 Yb or .sup.186 Re. Tc-99m is a preferred radionuclide because it emits gamma radiation at 140 keV, it has a physical half-life of 6 hours, and it is readily available on-site using a molybdenum-99/technetium-99m generator.
The sensitivity of imaging methods using radioactively-labeled peptides is much higher than other radiopharmaceuticals known in the art, since the specific binding of the radioactive peptide concentrates the radioactive signal over the area of interest. Small synthetic peptides that bind specifically to targets of interest may be advantageously used as the basis for radiotracers. This is because: 1. they may be synthesized chemically (as opposed to requiring their production in a biological system such as bacteria or mammalian cells, or their isolation from a biologically-derived substance such as a fragment of a protein); 2. they are small, hence non-target bound radiotracer is rapidly eliminated from the body, thereby reducing background (non-target) radioactivity and allowing good definition of the target; and 3. small peptides may be readily manipulated chemically to optimize their affinity for a particular binding site.
Small readily synthesized labeled peptide molecules are preferred as routinely-used radiopharmaceuticals. There is clearly a need for small synthetic labeled peptides that can be directly injected into a patient and will image pathological sites by localizing at such sites. Tc-99m labeled small synthetic peptides offer clear advantages as radiotracers for gamma scintigraphy, due to the properties of Tc-99m as a radionuclide for imaging and the utility of specific-binding small synthetic peptides as radiotracer molecules.
Radiolabeled peptides have been reported in the prior art.
Ege et al., U.S. Pat. No. 4,832,940 teach radiolabeled peptides for imaging localized T-lymphocytes.
Olexa et al., 1982, European Patent Application No. 823017009 disclose a pharmaceutically acceptable radiolabeled peptide selected from Fragment E.sub.1 isolated from cross-linked fibrin, Fragment E.sub.2 isolated from cross-linked fibrin, and peptides having an amino acid sequence intermediate between Fragments E.sub.1 and E.sub.2.
Ranby et al., 1988, PCT/US88/02276 disclose a method for detecting fibrin deposits in an animal comprising covalently binding a radiolabeled compound to fibrin.
Hadley et al., 1988, PCT/US88/03318 disclose a method for detecting a fibrin-platelet clot in vivo comprising the steps of (a) administering to a patient a labeled attenuated thrombolytic protein, wherein the label is selectively attached to a portion of the thrombolytic protein other than the fibrin binding domain; and (b) detecting the pattern of distribution of the labeled thrombolytic protein in the patient.
Lees et al., 1989, PCT/US89/01854 teach radiolabeled peptides for arterial imaging.
Sobel, 1989, PCT/US89/02656 discloses a method to locate the position of one or more thrombi in an animal using radiolabeled, enzymatically inactive tissue plasminogen activator.
Stuttle, 1990, PCT/GB90/00933 discloses radioactively labeled peptides containing from 3 to 10 amino acids comprising the sequence arginine-glycine-aspartic acid (RGD), capable of binding to an RGD binding site in vivo.
Maraganore et al., 1991, PCT/US90/04642 disclose a radiolabeled thrombus inhibitor comprising (a) a inhibitor moiety; (b) a linker moiety; and (c) and anion binding site moiety.
Rodwell et al., 1991, PCT/US91/03116 disclose conjugates of "molecular recognition units" with "effector domains".
Tubis et al., 1968, Int. J. Appl. Rad. Isot. 19: 835-840 describe labeling a peptide with technetium-99m.
Sundrehagen, 1983, Int. J. Appl. Rad. Isot. 34: 1003 describes labeling polypeptides with technetium-99m.
The use of chelating agents for radiolabeling polypeptides, and methods for labeling peptides and polypeptides with Tc-99m are known in the prior art and are disclosed in co-pending U.S. patent applications Ser. Nos. 07/653,012, 07/807,062, 07/851,074, and 07/871,282, which are hereby incorporated by reference.
Although optimal for radioimaging, the chemistry of Tc-99m has not been as thoroughly studied as the chemistry of other elements and for this reason methods of radiolabeling with technetium are not abundant. Tc-99m is normally obtained as Tc-99m pertechnetate (TcO.sub.4 ; technetium in the +7 oxidation state), usually from a molybdenum-99/technetium-99m generator. However, pertechnetate does not bind well to other compounds. Therefore, in order to radiolabel a peptide, Tc-99m pertechnetate must be converted to another form. Since technetium does not form a stable ion in aqueous solution, it must be held in such solutions in the form of a coordination complex that has sufficient kinetic and thermodynamic stability to prevent decomposition and resulting conversion of Tc-99m either to insoluble technetium dioxide or back to pertechnetate.
For the purpose of radiolabeling, it is particularly advantageous for the Tc-99m complex to be formed as a chelate in which all of the donor groups surrounding the technetium ion are provided by a single chelating ligand. This allows the chelated Tc-99m to be covalently bound to a peptide through a single linker between the chelator and the peptide.
These ligands are sometimes referred to as bifunctional chelating agents having a chelating portion and a linking portion. Such compounds are known in the prior art.
Byrne et al., U.S. Pat. No. 4,434,151 describe homocysteine thiolactone-derived bifunctional chelating agents that can couple radionuclides to terminal amino-containing compounds that are capable of localizing in an organ or tissue to be imaged.
Fritzberg, U.S. Pat. No. 4,444,690 describes a series of technetium-chelating agents based on 2,3-bis(mercaptoacetamido) propanoate.
Byrne et al., U.S. Pat. Nos. 4,571,430 describe novel homocysteine thiolactone bifunctional chelating agents for chelating radionuclides that can couple radionuclides to terminal amino-containing compounds that are capable of localizing in an organ or tissue to be imaged.
Byrne et al., U.S. Pat. Nos. 4,575,556 describe novel homocysteine thiolactone bifunctional chelating agents for chelating radionuclides that can couple radionuclides to terminal amino-containing compounds that are capable of localizing in an organ or tissue to be imaged.
Davison et al., U.S. Pat. No. 4,673,562 describe technetium chelating complexes of bisamido-bisthio-ligands and salts thereof, used primarily as renal function monitoring agents.
Nicolotti et al., U.S. Pat. No. 4,861,869 describe bifunctional coupling agents useful in forming conjugates with biological molecules such as antibodies.
Fritzberg et al., U.S. Pat. No. 4,965,392 describe various S-protected mercaptoacetylglycylglycine-based chelators for labeling proteins.
Fritzberg et al., European Patent Application No. 86100360.6 describe dithiol, diamino, or diamidocarboxylic acid or amine complexes useful for making technetium-labeled imaging agents.
Dean et al., 1989, PCT/US89/02634 describe bifunctional coupling agents for radiolabeling proteins and peptides.
Flanagan et al., European Patent Application No. 90306428.5 disclose Tc-99m labeling of synthetic peptide fragments via a set of organic chelating molecules.
Albert et al., European Patent Application No. WO 91/01144 disclose radioimaging using radiolabeled peptides related to growth factors, hormones, interferons and cytokines and comprised of a specific recognition peptide covalently linked to a radionuclide chelating group.
Dean, co-pending U.S. patent application Ser. No. 07/653,012 teaches reagents and methods for preparing peptides comprising a Tc-99m chelating group covalently linked to a specific binding peptide for radioimaging in vivo, and is hereby incorporated by reference.
Baidoo & Lever, 1990, Bioconjugate Chem. 1: 132-137 describe a method for labeling biomolecules using a bisamine bisthiol group that gives a cationic technetium complex.
It is possible to radiolabel a peptide by simply adding a thiol-containing moiety such as cysteine or mercaptoacetic acid. Such procedures have been described in the prior art.
Schochat et al., U.S. Pat. No. 5,061,641 disclose direct radiolabeling of proteins comprised of at least one "pendent" sulfhydryl group.
Dean et al., co-pending U.S. Patent Application 07/807,062 teach radiolabeling peptides via attached groups containing free thiols, and is incorporated herein by reference.
Goedemans et al., PCT Application No. WO 89/07456 describe radiolabeling proteins using cyclic thiol compounds, particularly 2-iminothiolane and derivatives.
Thornback et al., EPC Application No. 90402206.8 describe preparation and use of radiolabeled proteins or peptides using thiol-containing compounds, particularly 2-iminothiolane.
Stuttle, PCT Application No. WO 90/15818 describes Tc-99m labeling of RGD-containing oligopeptides.
Although it is possible to label specific-binding peptides with Tc-99m (as disclosed in co-pending U.S. patent applications Ser. No. 07/053,012, now abandoned, which issued as U.S. Pat. No. 5,654,272; Ser. No. 07/807,026, now U.S. Pat. No. 5,443,815; Ser. No. 07/851,074, now abandoned, which issued as U.S. Pat. No. 5,711,931; and Ser. No. 07/871,282, a divisional of which issued as U.S. Pat. No. 5,720,934, all of which are incorporated by reference), some such peptides exhibit low binding site affinity whereby the strength of peptide binding to the target site is insufficient to allow enough of the radioisotope to localize at the targeted site and form a radioimage. Peptides comprised of linear arrays of specific binding peptide repeating units have been described in the prior art. However, alternative arrangements of specific binding peptide units may be preferable in some cases.
Rodwell et al., 1991, PCT/US91/03116 disclose linear arrays of the peptide sequence RGD.
The present invention provides reagents comprised of a multiplicity of specific-binding peptide moieties, having an affinity for targeted sites in vivo sufficient to produce a scintigraphically-detectable image. The incorporation of a multiplicity of specific-binding peptide moieties in the reagents of the invention permits the use of specific binding peptides whose individual binding affinity would not otherwise be sufficient to produce a scintigraphically-detectable image in vivo. In other cases, an improvement in the scintigraphic image produced by a particular specific-binding peptide is achieved using the reagents of this invention.